Self-powered medical devices for treating cardiac fibrillation, such as portable external defibrillators and implantable cardioverter defibrillator (ICD) devices, are well known. These devices deliver a high voltage electrical countershock to the heart in an attempt to reset or restart normal cardiac rhythm. Typically, a set of high voltage capacitors in the device are charged via a transformer from a low voltage battery power source. The output of the high voltage capacitors is then discharged into at least two electrodes to create the high voltage electrical countershock. Preferably, the output of the high voltage capacitors is routed through a high voltage output switching network known as an H-bridge in order to create a truncated biphasic waveform. The H-bridge switching network creates the biphasic waveform by using at least four high voltage electronic switches connected between the capacitors and the electrodes and arranged in an H-configuration to switch the polarity of the output midway through the discharge of the capacitors. Examples of H-bridge switching networks for implantable defibrillators are shown in U.S. Pat. Nos. 4,800,883, 4,850,357, 4,998,531 and 5,083,562.
To date, the implementation of the H-bridge in a self-powered defibrillator has been accomplished using at least four high powered electronic switches, with the switches on the bottom or low voltage pole of the H-bridge configuration referred to as the low side switches and the switches on the top or high voltage pole of the H-bridge configuration are referred to as the high side switches. The H-bridge configuration is known for its ability to efficiently drive a load in two different directions from a DC voltage source. In a first phase, a first high side switch connects the high voltage pole (+HV) of the DC voltage source (the charged high voltage capacitors) to a first side of the load and a second low side switch connects a second side of the load to the low voltage pole (-HV) of the DC voltage source. To terminate the first phase, the second low side switch is turned off, thereby stopping current flow through both the first and second switches, after which the high side switch may be turned off. The second phase is initiated by a third high side switch that connects the high voltage pole (+HV) to the second side of the load and a fourth low side switch that connects the low voltage pole (-HV) to the first side of the load. The second phase is terminated similar to the first phase by turning off the fourth low side switch, after which current has stopped flowing through the load and the third high side switch may be turned off. For a more detailed description of the operation of the H-bridge circuitry of a defibrillator, reference is made to Bach, S. et al., "High Power Circuitry,"Implantable Cardioverter Defibrillator Therapy: The Engineering Clinical Interface, Chpt. 13, pp. 257-273, eds. Kroll, M. and Lehmann, M. (1996).
Because the switches in an H-bridge network must be turned on and off quickly and safely, existing designs for an H-bridge in a self-powered defibrillator use the two low side switches to control the operation of the H-bridge switch as just described. Originally, the low side switches were used for control of the H-bridge because the high side switches typically were silicon controlled rectifier (SCR) switches which cannot be turned off quickly or easily. As high power switches other than SCR switches (e.g., IGBT and MOSFET switches) have been used for the high side switches in an H-bridge a different problem known as hot switching has continued the reliance on the low side switches for controlling the operation of the H-bridge. Hot switching occurs when a switch turns on or off while passing current. The problem is that the switch must dissipate energy during hot switching. This dissipated energy is usually in the form of heat and is the product of the current through the switch and the voltage across the switch integrated over the transition time of the hot switching. If the voltages across the switch are large and the transition time is too long, it is possible for the dissipated heat to damage the switch. Because the low side switches are easier and quicker to control, the problems associated with hot switching are more manageable and the use of the low side switches to control the operation of the H-bridge switch has continued.
Even though the low side switches are used to control the H-bridge circuitry, the control voltages of 12-18 volts for the low side switches are still higher than the typically battery voltages of 3-9 volts. As a result, existing defibrillators typically include a voltage multiplier that increases the battery voltage to the voltage necessary to operate the low side switches. It has been found, however, that during periods of high current draw on the battery (such as when the battery is charging the capacitors), the ability of these voltage multiplier circuits to maintain the necessary voltages across the low side switches to prevent an unintended discharge can be compromised. U.S. Pat. No. 5,372,605 describes a dual battery system for an implantable defibrillator in which a booster circuit is incorporated into the design of the power circuitry of the device to insure that there is adequate voltage for controlling the low side switches, as well as other critical circuitry in the device, during periods of high current drain from the battery.
Because of the high voltages and high currents which are involved, it is also necessary to isolate the control circuitry for the H-bridge from the remaining circuitry in the device. Two alternative techniques have been used to accomplish this isolation. Either the high voltage capacitors and the H-bridge switching networks, including its control circuitry, can be completely isolated from the rest of the circuitry, or the negative pole of the high voltage capacitors can be connected to a common negative ground with the rest of the circuitry, thereby requiring only that the high side control signals for the H-bridge be isolated. Most of the H-bridge designs described above use the common negative ground technique and use some kind of pulse transformer or RF carrier with rectification to achieve the necessary high side isolation. U.S. Pat. No. 5,545,181 describes the other technique for isolating the H-bridge output switching network in an implantable defibrillator. In this patent, the low side of the H-bridge is referenced to a supply voltage which is negative with respect to the pacing and sensing ground for the remaining circuitry of the device and which is connected to the positive side of the low voltage battery for the device.
Other improvements have been made to the operation of the control circuitry for the H-bridge output switching network. In U.S. Pat. No. 5,178,140, capacitive coupling is taught for isolating the control circuitry for the H-bridge and includes a common mode switch for rejecting noise. In U.S. Pat. No. 5,470,341, the control circuitry for the H-bridge includes circuitry to inhibit high voltage transient noise signals that may be generated during switching of the H-bridge circuitry so as to prevent inadvertent retriggering of the control circuitry. In U.S. Pat. No. 5,534,814, a high impedance gate driver circuit is utilized as part of the control circuitry in order to minimize the amount of base current needed to drive the IGBT high power switch which forms the H-bridge switching network in this patent.
In U.S. Pat. No. 4,823,796, a pair of optoisolators are used for isolation of a voltage detection circuit and an energy selection circuit for the output switching network of a self-powered external defibrillator. This patent otherwise uses conventional designs for isolating the high voltage output switching network from the remaining circuitry of the device and for controlling the switching operation of the output switching network.
In U.S. Pat. No. 5,626,619, an optically isolated shock control circuit is described. In this patent, several alternate embodiments of a high side and low side control circuits for the H-bridge are presented in which the control signal is transmitted by an optical phototransistors or photodiodes. This patent specifically describes how the optically isolated control circuit can be implemented with N-channel IGBT or MOSFET switches, instead of the conventional SCR switches. While the invention described in this patent represents a significant advance from previous isolated control circuits due to the use of the phototransistor as the isolation mechanism and the use of N-channel switches, the practical embodiment of the invention still relies on low side control of the H-bridge.
In U.S. Pat. No. 5,674,266, a diode isolated shock delivered circuit is described for a self-powered external defibrillation. In this patent, a four diode bridge having an input node and output node is used to isolate the H-bridge circuitry from the patient. The control circuitry for this design is driven by a conventional gate drive circuit that is controlled by an isolated pulse control signal.
In U.S. Pat. No. 5,693,952, both a switch-on optoisolator and a switch-off optoisolator are used as isolation and control for the H-bridge circuitry. In one embodiment, the performance of the switch-on function is enhanced by using a first optoisolator to charge a small capacitor and then using a second optoisolator to discharge that capacitor to operate the high voltage switch in the H-bridge circuit.
U.S Pat. Nos. 4,902,901, 5,360,979 and 5,532,498 describe various control circuitry for optically controlled solid state switches that have improved switching performance. In each of these patents, isolation and control is accomplished using a photo-diode array as the current source for controlling the operation of the solid state switches. However, none of these circuits are specific to the requirements of the H-bridge circuitry within a self-powered defibrillator.
While the existing designs for the H-bridge and power circuitry for self-powered defibrillators are adequate, it would be desirable to provide improvements which could increase the performance efficiency of the defibrillator and which could allow for more flexibility in the operation of the defibrillator.